Casters, which generally comprise a wheel or the like swivel-mounted on a chassis, are used in many industrial applications. They can be ordered with different configurations, sizes, materials, and load capacities. This invention relates a caster assembly held together by a drawn kingpin rivet. The assembly exhibits several unique qualities, including the ability to passively absorb shock to a much greater extent than similar devices.
There are three general categories of casters: furniture, industrial, and specialty. Furniture casters are found on all types of office and household items. They are the cheap to produce, as they are largely constructed from injection molded plastic, but they suffer from a relatively low load capacity (<100 lbs.)
Industrial casters are generally manufactured by an economical stamping or cold forging process with secondary welding operations. They include light-duty casters, such as those found on shopping carts, to heavy-duty casters, such as those found on sophisticated manufacturing equipment. The price for industrial casters ranges from $2–3 for light-duty casters to well over $100 for heavy-duty casters.
As the name implies, specialty casters are designed for specific industrial applications and include super-heavy-duty casters (for loads greater than 2,000 lbs) and shock absorbing casters. Not surprisingly, specialty casters are usually significantly more expensive than standard industrial casters.
Industrial casters that include a mounting plate, which facilitates attachment to equipment, are usually constructed in one of three distinct ways that vary greatly in cost, quality, and durability. The cheapest and most widespread construction has stamped or cold forged sheet metal parts held together by a kingpin rivet. The sheet metal parts are typically produced from a low carbon steel, such as hot roll strip, that is inexpensive and easy to form due to its high ductility. However, low carbon steel can tolerate very little strain prior to yield and has a low toughness in addition to other poor mechanical properties. Accordingly, in medium to heavy-duty applications, caster parts produced from this material tend to be very thick and heavy, and the low rate of work-hardening results in a surface hardness that is insufficient to handle significant bearing loads. Casters of a higher quality using this construction will therefore case harden the parts that come into contact with the hardened ball bearings to prolong the life of the bearing surfaces and increase the overall structural rigidity of the caster. Case hardening encases the part with a thin martensitic layer of steel that is much harder than the internal steel matrix. The depth of this layer is referred to as the case depth. The case-hardened parts are subsequently rack-plated with a bright zinc finish or even a chrome finish to prevent atmospheric oxidation.
The kingpin rivet that holds these parts together is typically an elongated cylindrical part with a wider head portion. Such a kingpin can be inexpensively mass-produced from a mild steel by a screw machining process or cold-headed much like the head of a conventional bolt. The kingpin rivet is inserted though a central aperture in all the parts and the end opposite the head is mechanically or hydraulically swaged outwards to permanently retain the assembly. The strength of the kingpin rivet increases dramatically as the diameter increases with typical diameters ranging from 7/16–¾ of an inch.
Under extreme conditions, casters of any kind may exhibit various modes of failure. “Soft” modes of failure are the most common, and generally precede “hard” or catastrophic failure. Soft failure modes include brinelling of the raceways where the hardened steel ball bearings create small indentations in the metal, “flat spotting” of the wheels where the material creeps under an excessive load, or severe corrosion. None of the soft failure modes necessarily renders the caster assembly unusable as is the case with hard or catastrophic failure.
Industrial casters with kingpins and case hardened sheet metal parts serve the majority of industrial caster applications but are notorious for suffering catastrophic kingpin failure. During movement, all kingpins can be subjected to extreme degrees of stress as static and dynamic loads combine to form strong moments that act about them. Under these conditions, the portion of the kingpin opposite the wheels comes under the greatest stress particularly in the area where it has been swaged. Overstressing a swaged kingpin in this area may lead to a soft failure deformation of the metal, wherein the assembly is no longer bound tightly together, or an abrupt catastrophic failure. Over the years, manufacturers have strengthened kingpins and in some cases have replaced them altogether with a commercial grade nut and bolt that offers greater strength but adds more cost. In either case, this shifts the point of catastrophic failure to other parts in the assembly.
The second industrial caster construction utilizes a kingpin stud that is integrally hot-forged with the mounting plate and is generally reserved for the heaviest duty applications (2,000–20,000 lbs). Hot forged casters are manufactured from medium carbon steel that is heated to elevated temperatures and worked into shape by a forging press. The forged parts are extremely strong and include the mounting plate with the integral kingpin and upper race and a lower bearing raceway. A drawback of the hot forging process is that close tolerances cannot be held and smooth finishes cannot be controlled. This is critical for any bearing raceway or precision bearing bore. Therefore, subsequent machining operations must be performed which add considerable cost to the parts. After the machining process is completed, the ball bearing raceways are hardened to make them capable of handling the heaviest loads and welding operations are performed to affix the caster legs to the lower race. After powder coating them for good cosmetic appearance and corrosion resistance, the assembly is ready for final assembly. This is completed when a large king nut is tightened on the integral kingpin stud of the mounting plate. Hot-forged casters constructed in this fashion are far more expensive than sheet metal kingpin casters but are considered the most durable and robust construction.
The third construction eliminates the kingpin altogether and is commonly referred to as “kingpinless.” Exactly how the kingpin is eliminated varies, but this construction generally combines cold forged parts with machined parts or hot-forged parts together in a welded assembly. One such implementation has a rectangular mounting plate welded to a tubular inner race that has a single ball-bearing groove machined about the outside diameter. The outer raceway has an inside diameter slightly larger than the outside diameter of the inner race and has a complementary bearing groove machined into its inner diameter. On the top end, the outer race is spaced to clearance the mounting plate and the caster legs are welded to the bottom end. The unit is assembled by aligning the inner and outer raceways and then inserting the hardened ball bearings through an access hole drilled in the outer race. This hole is subsequently plugged or sealed with a welding operation. The resulting caster assembly is superior in strength and durability to a comparable kingpin caster because the assembly is retained about a much larger diameter by the hardened ball bearings and raceways. However, the machined parts and extra assembly required adds considerably to the cost of the unit.
These three industrial caster constructions are designed for general material handing applications and offer very little in the way of shock absorption. When casters do encounter obstacles in the course of ordinary duty, most of the shock is transmitted directly through to the supported equipment. These shock loads also substantially and momentarily increase the load upon the caster. For reference, this dynamic load can easily be twice the static load and a typical duration of this load may be 30 milliseconds. It is not surprising then that most casters experience soft failure bearing damage or catastrophic failure under these service conditions.
Most caster manufacturers offer specialty shock-absorbing casters that contain active elements for shock absorption that are not found on ordinary industrial casters for use in application where shock is an issue. The active elements may include compressible elastomer pads, die springs and the like that are selected to absorb shock over a particular range of loads. These casters are well-suited for use with sensitive electronic hardware and equipment, but they cost about four times more than an ordinary caster with similar load capacities. As a result, they are used only in applications where cost is not an issue or those where they are absolutely necessary.
At least one manufacturer offers an elastomer wheel with a unique circular pattern of geometrical cavities. This is an example of passive shock absorption where an existing caster part or component is modified for improved shock absorbing capability. Compared to active shock absorption, passive shock absorption generally does not perform as well but it is far less expensive.
As noted above, low carbon steel is the typical material for industrial casters. Stainless Steel industrial casters are available for corrosive service conditions or frequent wash-down applications, but because stainless steel by weight costs five to six times as much as low carbon steel and it is difficult to machine, generally only stamped stainless casters with kingpin rivets are available commercially. Hot-forged stainless casters with integral kingpins do exist; however, they are extremely expensive. The vast majority of stamped stainless casters are currently made from an annealed 304 series stainless steel. In this condition, the material is about 20% stronger and considerably harder than the low carbon steel used in ordinary casters. The high nickel alloy (8–10%) content increases ductility for a high degree formability. Cold working low carbon steel and 304 stainless will both lead to increased strength and decreased ductility. However, for a given amount of cold work, the rate of work-hardening is far higher in the stainless steel. This property of metals to combine high strength with good ductility is a material property known as toughness.
As the material is formed into the shape of the caster parts, it is moderately work hardened to achieve a higher level of strength and hardness. Because 300 series austenitic stainless steels cannot be heat treated, the surface hardness of the bearing raceways is substantially less than that of case hardened low carbon steel parts. However, because strain hardening occurs throughout the parts as opposed to a limited case depth, the raceways can almost support as much as without bearing indentation. Finally, the complexity of the dies to manufacture stainless caster parts is considerably higher requiring a greater degree of expense and expertise. It should be noted that a die designed for stainless can also be used to run softer materials including low carbon steel. This is common practice in the industry so often the stainless parts are just as thick and heavy as the low carbon parts while costing much more.